In many instances it is desired to coat the surface of an object with a plurality of distinct, superposed layers (collectively, the plurality of layers is also known as a coating pack). For example, a common commercial operation involves application of a plurality of paint coatings to an article. Another common example is the manufacture of photographic elements, such as photographic film or paper, wherein a number of layers (up to ten or more) of different photographic coating compositions must be applied to a suitable support in a distinct layered relationship The uniformity of thickness of each layer in the photographic element must be controlled within very small tolerances.
Common methods of applying photographic coating compositions to suitable supports involve simultaneously applying the superposed layers to the support. Typically, a coating pack having a plurality of distinct layers in face-to-face contact is formed and deposited on the object so that all the distinct layers are applied in a single coating operation. In the photographic industry, several such coating operations may be performed to produce a single photographic element. Several methods and apparatus have been developed to coat a plurality of layers in a single coating operation. One such method is by forming a free falling, vertical curtain of coating liquid which is deposited as a layer on a moving support Exemplary "curtain coating" methods of this type are disclosed in U.S. Pat. Nos. 3,508,947 to Hughes, 3,632,374 to Grieller, and 4,830,887 to Reiter
"Bead coating" is another method of applying a plurality of layers to a support in a single coating operation. In typical bead coating techniques, a thin liquid bridge (a "bead") of the plurality of layers is formed between, for example, a slide hopper and a moving web. The web picks up the plurality of layers simultaneously, in proper orientation, and with substantially no mixing between the layers Bead coating methods and apparatus are disclosed, for example, in U.S. Pat. Nos. 2,681,294 and 2,289,798.
In both bead coating and curtain coating methods, it is necessary to set and/or dry the layered coating after it has been applied to the support. To accomplish this, the web is typically conveyed from the coating application point to a chill section. Subsequently, the web is conveyed through a series of drying chambers after which it is wrapped on a winder roll. Space constraints for the coating machine, cost considerations, and flexibility of design may dictate that one or more inclined web paths be present in conveying the coated substrate from the coating point to the chill section and drying chambers.
Advancements in coating technology have led to increased numbers of layers coated at each coating station, increased total pack thickness per station, thinner individual layers, use of rheology-modifying agents, and the development of new, sophisticated chemistries. In addition, a multilayer photographic coating can consist of sensitizing layers and/or additional, non-imaging, layers. As a result, the chemical composition of the multilayer coating pack is often markedly different from one layer to the next.
In accordance with the present invention, it has been discovered that the above-mentioned factors, in conjunction with the use of web paths implementing vertical components (inclines), has led to the development of a certain, specific nonuniformity in the coated layers. It has been found that this nonuniformity, referred to herein as "ripple" or "ripple imperfection", is caused by interfacial wave growth in the flow of a multilayer coating on the web. Ideally, the flow of the layers on the web is plug (i.e., all layers, as well as the web, are moving at the same speed). However, it has been found in accordance with the present invention that inclined web conveyance ,paths facilitate a gravity-induced flow of the layers relative to the web. This gravity-induced flow supports the existence of waves which increase in amplitude as the layers translate with the web. It is believed that this wave growth is manifested as "ripple".
The causes of and solutions to the problem of ripple imperfections in multilayer coatings have gone largely unexplored. The present invention addresses this problem and discloses a method of reducing the likelihood and severity of ripple formation in coating multilayer liquid packs.